31 May 2016

Future of Rotary Wing Craft

By Gp Capt AK Sachdev
30 May , 2016

Technological advances have brought impressive improvements in rotorcraft technology albeit without eliminating the intrinsic limitations. The classic helicopter design has been mutated into newer types of rotorcraft or hybrid VTOL ones. Nonetheless, the majority of helicopters retain the original tadpole-like shape with a main rotor and a tail rotor. Designers all over the world are persistently and continually seeking breakthroughs to achieve incremental performance parameters. Advances in high technology regimes continue to provide slow but steady support.

Size has always been a laudable and desirable criterion for any craft…

The term ‘pecking order’, so visibly manifest in chickens, supposedly owes its origin to the Red Jungle Fowl of Thailand that evolved it as an aid to survival. It ensured that, when limited food was available, there were no fights over food which could attract attention of predators to the flock. In the technological marvels of the 20th century, fowl created from metals and alloys have displayed a similar, apparent ‘social’ demeanour wherein resources have been served up to the stronger of the flying machines. Thus, fixed wing combat aircraft with higher speeds and offensive roles have usurped a slot higher in the pecking order than slower, less potent, ground hugging ones, the helicopters.

Witness the fact that the first production helicopter, the Sikorsky R-4 in 1942, came over three decades after the first mass produced fixed-wing aircraft the Wright Model B in 1910. That is not to say that the potential of rotary wing flight was not recognised early. Indeed, US National Aeronautics and Space Administration (NASA) has had its Langley Research Centre in Hampton since 1917, where it has been carrying out research on helicopters and other vertical flight aircraft for almost a century. The Centre was recently named the Vertical Flight Heritage Site by the American Helicopter Society International (AHSI), the world’s only international society for engineers, scientists and others working on vertical flight technology. However, traditionally, fixed-wing R&D has arrogated a larger share of the funding pie than the rotary wing domain.

The aviation enthusiast, civil or military, tends to the predisposition that helicopter performance has reached a plateau and that future developments are unlikely to demonstrate spectacular advances in speed, performance or design. While fixed wing advances may be dramatically projected through the fifth-generation F22 Raptor with several others said to be under development, progressions on the rotary wing front are also worthy of attention, certainly more than they receive now.

The heavier the helicopter, the more restricted its operational envelope…

Design Dynamics

At the beginning of World War II, Igor Sikorsky flew the Vought-Sikorsky VS-300. Its modified version, the Sikorsky R4 became the world’s first mass-produced helicopter in 1942 and saw limited employment during the War. A look at a picture of the R4 will confirm that basic helicopter profile has more or less remained the same over seven decades although design and capability have struggled to keep up with mission and role requirements. Design R&D endeavours have continually addressed the inherent imperfections of the helicopter, the prominent ones being restricted payload, poor performance, limited speed, excessive noise, high vibration levels, high vulnerability and low survivability.

The 1920s saw significant changes in helicopter design with application of aerodynamic theory to rotor systems, availability of high power-to-weight ratio aero engines capable of supporting vertical flight and lowered cost of aluminium alloys for light-weight airframes. Initial challenges to helicopter design were counteracting main rotor torque reaction, providing safe levels of stability and control and reducing vibrations to acceptable levels. Once these had been tackled, rotorcraft design graduated to higher levels of sophistication and evolved from the original single main rotor to tandem rotors, synchropter (intermeshing rotors), co-axial rotors, tip-jet driven rotorcraft, No Tail Rotor (NOTAR), tilt-wing aircraft, tilt-rotor aircraft and compound helicopters.

Big is Beautiful

Size has always been a laudable and desirable criterion for any craft. The largest helicopter ever flown was the V12, built in late 1960s by the Mil Moscow Helicopter Plant. The company merged with Kamov and Rostvertol to form Oboronprom Corporation in 2006. The helicopter had a Maximum Takeoff Weight (MTOW) of 105 tonnes. Only two prototypes were built, but they still hold four current world records for payload/altitude combinations. The Mil Mi-26, with an MTOW of 56 tonnes is the largest, heaviest and most powerful helicopter to go into series production. While the Mi-26 had a unique design with a single disc having an eight-blade rotor, the closest Western heavy-lift helicopter is the CH 53 Super Stallion (MTOW 33.3 tonnes) followed by CH47 Chinook (MTOW 22.7 tonnes). The CH53 has a seven-blade rotor and the CH47 has a tandem rotor system.

The foregoing figures have been mentioned to assert that, as far as designer’s skill and operator’s ambition are concerned, big can keep becoming bigger. However, the heavier the helicopter, the more restricted its operational envelope including the ground infrastructure and helipad size. As an illustration, a Chinook being operated by US 160th Special Operations Aviation Regiment was stranded at 2,600 metres in Afghanistan; stripped of fuel, rotors and all non-essentials, it weighed an estimated 12 tonnes. As the largest US helicopter, the Super Stallion could carry only nine tonnes at that altitude, a civilian Mi26 was leased and the Chinook airlifted to Bagram Air Base. India has used the Mi26 in the past and is in the process of procuring the Chinook.

The helicopter is a complex machine which uses a substantial part of the lift its rotor system generates to just stay away from the ground…

As an aside, Russian Helicopters and Aviation Industry Corporation of China (AVIC) have signed a framework agreement to work together on creating an Advanced Heavy Lift (AHL) with take-off weight of 38 tonnes and the capability to carry ten tonnes of cargo inside the cabin or 15 tonnes under-slung. The helicopter will be designed to operate round-the-clock in hot climates, mountainous terrain and all weather conditions and will be able to fly a highly varied range of missions.

Material

The helicopter is a complex machine which uses a substantial part of the lift its rotor system generates to just stay away from the ground even when not using its engine power to move from one place to another. Thus weight becomes a crucial design factor. Interestingly, the first flying aircraft of the Wright Brothers had a wooden airframe, but the engine crankcase was made of aluminium to keep weight down. Aluminium gradually replaced wood, steel and other parts in the early 1900s and the first all-aluminium plane was built in the early 1920s. Since then, aluminium alloys have become critical to airframe manufacture and today represent about 80 per cent of a typical helicopter airframe.

With significant advances in carbon composites, lighter weight with ever increasing strength has become possible and every helicopter manufacturer is increasingly eyeing carbon composites although the cost factor has bridled that trend a bit. New Zealand’s Composite Helicopters International has been developing the KC518 Adventourer, an all-composite, frameless, six-seat helicopter constructed from Carbon and Kevlar using EvoStrength technology. It is the world’s first helicopter with a monocoque fuselage made entirely from composite materials and is a change from the use of aluminium and steel tube framing used in helicopter manufacture.

As a single piece composite structure, there are no rivets or bolts used in the assembly with the result that there is substantial resistance to corrosion, fatigue and impact. A possible threat to the programme, a recent accident where the prototype pitched up and rolled on its side during an emergency landing, was converted into an opportunity for promotion by the company. The company President Peter Maloney was quoted as saying, “Though the accident was unfortunate, we are impressed by the strength of the carbon-Kevlar composite fuselage which withstood the force of impact when landing. It was the unique composite structure of the helicopter that saved us from serious injury.”

The single major limitation of a helicopter is its forward speed…

Need for Speed

The single major limitation of a helicopter is its forward speed. It would be stating the obvious that a faster helicopter would be more desirable and designers have indeed been fervently working towards helicopters that can fly faster. While the drag reduction logic applicable to fixed-wing airframes apply to a large degree to a helicopter, there is an insurmountable problem associated with the rotor disc.

The geometry of a typical helicopter rotor has the inherent aerodynamic challenge with the rotor system which increases with forward speed. During hover, all the rotors travel at a velocity determined by the length of the blade and the Revolutions Per Minute (RPM). However, as the helicopter moves forward, the velocity of the blades relative to the air depends on the velocity of the helicopter as well as the rotor speed. The airspeed of the advancing rotor blade is much higher than that of the helicopter itself and it tends to approach the speed of sound with resultant increased drag and vibration. Also, because the advancing blade has higher velocity than the retreating blade, there is a asymmetry of lift between the two, rotor blades are designed to ‘flap’ i.e. lift and twist in such a way that the advancing blade flaps up and develops a smaller angle of attack. Conversely, the retreating blade flaps down, develops a higher angle of attack and generates more lift.

At high speeds, the force on the rotors is such that they ‘flap’ excessively and the retreating blade can reach too high an angle and stall. The forward speed is thus limited and any endeavour to go beyond around 130 knots, involves special materials and technologies. In a tandem rotor system, like the Chinook, the effect of retreating blade lift asymmetry on one rotor is countered to some extent by the other, but not wholly.

The leaders in the race for faster helicopters are the Sikorsky X2 and the Eurocopter X3. Reportedly, in June 2013, the X3 broke the X2 records for level flight and dive speed, recording 255 knots and 263 knots respectively. However, it may be mentioned that both X2 and X3 are technology demonstrators and are not designed as classic helicopters. The X3 has stub wings fitted with a tractor propeller at each tip. The differential propeller pitch counters the torque effect of the main rotor.

The traditional helicopter design would not quench operators’ thirst for higher speeds and better performance…

The X2, on the other hand, uses co-axial rotors with a pusher propeller at the tail. The X2 and X3 records are unofficial as they are not recognised by World Air Sports Federation which recognises a 1986 record of 249 knots set by a Westland Lynx demonstrator. Russia is also reportedly working on a high speed design with a target of bettering 249 knots. The prototype based on a Mi24 airframe is expected to be ready to fly by 2020 and a flying laboratory is expected to be set up by end-2015 under the programme named Perspectivny Skorostnoy Vertolyot (PSV) or advanced high-speed helicopter.

It is worth a mention here that the marginal increase of speed that rival companies are so assiduously working to achieve is essentially a public relations exercise and comes at enormous cost. Thus, while breaking speed records is one thing, finding civil or military operators willing to pay very high costs to get a few extra knots may be very difficult. Witness the announcement of Eurocopter X4 in 2011 which has now been unveiled as Airbus H160. While the X4 nomenclature indicated a progression beyond X3, the H160, with a cruising speed of 160 knots, has no pretensions to record breaking speeds.

Vertical Take Off and Landing (VTOL)
From the foregoing, it would be obvious that the traditional helicopter design would not quench operators’ thirst for higher speeds and better performance. The basic design has other limitations as well. The rotor system is not as efficient for forward travel as a fixed-wing one, uses more fuel and requires more maintenance. It consumes approximately 15 per cent of its total power available to run the essential tail anti-torque rotor just to keep the helicopter from spinning. The helicopter must also deal with high vibration levels.

There are many other safety related problems that beset a helicopter design; some of them are loss of tail rotor effectiveness, ground resonance, mast bumping, loss of control during negative ‘g’ conditions, power settling, dynamic roll-over, anti-torque rotor failures, the requirement for unusually quick response in case of engine failure and the non-feasibility of an ejection system for the crew. Thus, over the last few years, it was realised that there was a need to move away from the original shape but retain the Vertical Take Off and Landing (VTOL) characteristic of a helicopter. Developing a practical, hybrid aircraft with the performance of a fixed wing aircraft in forward flight has proven to be a surprisingly difficult task.

Bell Helicopter has been dominant in tilt-rotor development with major designs from almost every decade back to the 1950s…

The engineering challenge consisted of achieving two main goals. The first was to accomplish controllable vertical flight using the very same mechanisms and equipment that are required for forward flight. Any weight of exclusively vertical flight mechanisms would be useless during forward flight and would represent a reduction in available payload relative to a comparable fixed wing aircraft capability. The second goal consisted of achieving ‘power matching’ i.e. a VTOL design that requires the same power in vertical flight as in forward flight. Any mismatch would represent excess capacity which corresponds to excess weight in one mode of flight. Numerous approaches to VTOL aircraft have been explored over the years; the prominent ones are tilt-rotors, tilt-props and tilt-wings, as well as deflected-slipstreams, deflected-thrust, thrust augmenters, ducted fans, tilt ducted rotors and tail sitters.

As the name implies, a tilt-rotor aircraft uses tiltable (rotating) propellers, or proprotors, for lift and propulsion. For vertical flight, the proprotors are angled to direct their thrust downwards, providing lift. In this mode of operation the aircraft is essentially identical to a helicopter. As it gains speed, the proprotors are slowly tilted forward, eventually becoming perpendicular to the ground. In this mode the wing provides the lift, and the wing’s greater efficiency helps the tilt-rotor achieve its high speed. In this mode, the platform is essentially a turboprop aircraft.

Bell Helicopter has been dominant in tilt-rotor development with major designs from almost every decade back to the 1950s. They are currently partnered with Boeing on the first production tilt-rotor aircraft, the jointly developed and manufactured Bell/Boeing V22 Osprey. Tilt-rotor proprotors require all the fundamental parts of a twin-rotor helicopter. They also have a full set of airplane controls and a tilt mechanism that rotates the lifting rotors while carrying flight loads. This means that the cost of a tilt rotor is typically 50 to 100 per cent more than a helicopter of the same power and empty weight.

The Bell V-280 Valor is a tilt-rotor concept being developed by Bell Helicopter and Lockheed Martin for the US Army…

Incidentally, India’s Aviation Research Centre (ARC), responsible for electronic surveillance and signals intelligence along the country’s borders with Pakistan and China, wants to acquire Bell-Boeing V22 Osprey tilt-rotor aircraft to augment its operational capabilities. According to IHS Jane’s, Bell-Boeing has recently provided ARC with price estimates and delivery schedules for four Ospreys to be acquired via the US Foreign Military Sales route. The ARC Ospreys could be used to deploy Special Frontier Force personnel and provide them logistic support on missions in tandem with Research and Analysis Wing (RAW), India’s external intelligence-gathering agency.

The Chinese toy industry has popularised the quadcopter globally. However, aircraft designers in pursuit of VTOL technology have found that the quadcopter does not perform impressively when scaled up to a size with significant payload. Bell and Boeing are working on larger Quad Tilt Rotor (QTR) military models for possible use by the US Army. The QTR has two sets of fixed wings and four tilting rotors mounted at the tips of the wings. The program has been nicknamed the V44 Tilt Rotor for the four tilt rotor version and V66 Tilt Rotor for the six tilt-rotor version. These poly-tilt-rotor aircraft have also been called “The Flying Freight Train” for their large capacity almost the size of a railcar which could carry as many as 100 passengers or troops or heavy cargo over 50,000 pounds. They would use advanced versions of the tilt-rotor engines used for the V-22 Osprey.

The UK-based aerospace company AgustaWestland is developing AW609 formerly known as Bell/Agusta BA609, for civilian use. In an allied development endeavour called Project Zero, AgustaWestland is working on the world’s first electric tilt-rotor aircraft, as a technology demonstrator termed Project Zero. Each of its two rotors is driven by its own electric motor, which is powered by rechargeable batteries. The aircraft’s control systems, flight controls and landing gear actuators are also all electrically powered, so there is no need of the hydraulic system. Besides, the aircraft doesn’t require a transmission as well. A first flight was demonstrated in early 2013, but a dateline for production is yet to be announced. A hybrid version, which would use a diesel engine to power a generator, is under consideration.

The K-MAX is a UAS transformed by Lockheed Martin Corporation and Kaman Aerospace Corporation…

NASA’s Greased Lightning or GL-10 deserves a mention here. It is a battery-powered, ten-engine remotely piloted tilt-rotor and the prototype has a ten feet wing span and can take off vertically like a helicopter as also fly efficiently in forward flight. It is in design and testing phase and flew a series of test flights during early May 2015. The final version is expected to have a 20-foot wing span.

The Bell V-280 Valor is a tilt-rotor concept being developed by Bell Helicopter and Lockheed Martin for the US Army. In one major difference from the earlier V22 Osprey tilt-rotor, the engines remain in place while the rotors and drive shafts tilt. A driveshaft runs through the straight wing, allowing both proprotors to be driven by a single engine in the event of engine loss.

Besides tilt rotors, VTOL aircraft could have other designs like ducted fans Bell X22A, Ryan XV5A/B, hovering platforms UrbanAero X-Hawk, or the Elytron design which combines three sets of wings and one pair of rotary wings called ‘proprotors’, mounted on a single tilt-wing in central position and two pairs of fixed wings or the Disc Rotor in which for hover, a set of blades are extended from the periphery of the disc, much like a helicopter but forward flight is like a fixed wing aircraft with the blades either fully retracted into the disc or with two of the rotors sticking out like conventional lift producing wings.

Unmanned Craft

While toy helicopters, especially quadcopters, are easy to construct, practical drone helicopters with useful payload become increasingly complex to design. The maximum and most interesting variations of helicopter designs are emerging in the Unmanned Aircraft System (UAS) regime. The GL10 mentioned earlier is a small tilt-rotor but there is a spectacularly successful drone helicopter that has pioneered its way to fame.

The K-MAX is a UAS transformed by Lockheed Martin Corporation and Kaman Aerospace Corporation from an existing helicopter model of Kaman to enable US Marines to deliver supplies by day or night to precise locations without risking lives. The aircraft can fly at higher altitudes with a larger payload than any other rotary wing UAS. It can lift and deliver 2,700 kg of cargo at sea level and over 1,800 kg at 15,000 feet density altitude. With its four hook carousel, the K-MAX UAS can also deliver more cargo to more locations in one flight in autonomous or remotely controlled operations. The K-MAX did laudable service in Afghanistan and has proven the concept beyond any reasonable doubt. The future can be expected to see more variations of the concept in optionally manned versions as well.

The Mars Helicopter would revolutionise the manner in which future Mars exploration could progress…

Another noteworthy helicopter innovation is the Mars Helicopter being developed by NASA. A team at NASA’s Jet Propulsion Laboratory is working on a prototype, autonomous, solar powered helicopter that would work as a scout for a rover, giving NASA teams a heads-up to what they might encounter ahead and steering them toward the best possible location. Due to the large distance from Mars, remote control is not technically feasible in real-time as it would take minutes for radio waves to travel one way between Mars and any control station on Earth. Therefore, the Mars Helicopter would be totally autonomous. The Mars Helicopter would revolutionise the manner in which future Mars exploration could progress and is expected to be ready well in time for the Mars 2020 Rover.

US Future Vertical Lift Programme

It is not possible to describe all the developments going on in vertical lift technology across the world but it is worth mentioning the Future Vertical Lift (FVL) programme of the US Department of Defence (DoD) because of the focussed approach it adopts towards the problem of new types of rotorcraft for US Army beyond 2030. In October 2011, DoD issued the FVL Strategic Plan to outline a joint approach for the next generation vertical lift aircraft for all military services.

The FVL concept is to create new rotorcraft using new technology, materials and designs that are quicker, have further range, better payload, are more reliable, easier to maintain and operate, have lower operating costs and can reduce logistical footprints. FVL is to create a family of systems to replace most Army helicopters. A precursor for FVL is the Joint Multi-Role (JMR) helicopter programme, which will provide technology demonstrations planned for 2017. JMR Phase I will develop the air vehicle while JMR Phase II will develop mission systems.

The FVL concept is to create new rotorcraft using new technology, materials and designs…

The US Army plans to acquire as many as 4,000 aircraft from the FVL programme as a replacement for its UH-60 Black Hawk, AH-64 Apache, CH-47 Chinook and OH-58 Kiowa helicopters. The main contenders are Bell Helicopter, Boeing/ Sikorsky (jointly), AVX Aircraft, EADS North America, Piasecki and Karem Aircraft. In tandem, the US Army’s Improved Turbine Engine Program (ITEP) is aimed at a 2023 production goal. Advanced Turbine Engine Company (ATEC), a 50-50 joint venture of Pratt & Whitney and Honeywell, is competing with GE Aviation to develop a drop-in replacement for the legacy GE T700 engines that power the Boeing AH64 Apache and Sikorsky UH60 Black Hawk fleets. The engine is also expected to power light rotary-winged aircraft expected to emerge from the FVL programme.

Conclusion

Technological advances have brought impressive improvements in rotorcraft technology albeit without eliminating the intrinsic limitations. The classic helicopter design has been mutated into newer types of rotorcraft or hybrid VTOL ones. Nonetheless, the majority of helicopters retain the original tadpole-like shape with a main rotor and a tail rotor. Designers all over the world are persistently and continually seeking breakthroughs to achieve incremental performance parameters. Advances in high technology regimes continue to provide slow but steady support.

Upgrades cost a fraction of the cost of a newer helicopter although the upgraded version may not turn into a new helicopter in terms of performance…

For the helicopter operator, technological progression holds out two different but related promises. The first concerns new and innovative changes to rotorcraft design and is predicated to new projects and enterprises. We have seen glimpses of such novel inventions in the foregoing narrative.

However, there is another way in which technological advances are becoming expedient for the helicopter industry and that is the increasing capability to upgrade existing models so as to increase their performance, role capability and utilisation for new missions. Not only that, upgrades can also prolong the life of a particular model and render it safer for example, by reduction in vibration levels.

Needless to say, upgrades cost a fraction of the cost of a newer helicopter although the upgraded version may not turn into a new helicopter in terms of performance. It would appear that, at least for a decade more, upgrades of existing, tried-and-tested models are likely to be at least as prominent as the emergence of new machines.
© Copyright 2016 Indian Defence Review

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